Systems (WGS) developed by the Defense Mapping Agency are all global coverage datums; WGS 84 being the newest and most accurate.  A World Geodetic System offers the basic geometric figure of the earth (ellipsoid) as well as an associated gravity model (geoid).  Because of this, the geoid separation remains relatively small over the entire system (generally less than 102 meters within WGS 84).     ELLIPSOID EARTHS SURFACE Datum Point Figure 2-11  Earth-centered Earth-fixed Ellipsoid 2-14  Multiple Datum Problems a.  Over 1000 different datums exist.  Practically every island or island group in the Pacific Ocean has its own datum; many areas of the world are covered by multiple datums.  This causes the most concern for surveyors who must decide which datum to use, and then how to convert data between them.  Mapping products established from different datums will not match at the neatlines, nor will grid lines meet.  Target Acquisition assets will provide inaccurate data to firing systems if the target acquisition system is not on the same datum as the firing system. b.  The WGS was developed to create a global system which could be used to alleviate many of these problems. See Appendix A-2.  The National Imagery and Mapping Agency (NIMA) is working to revise all mapping and charting products to reference WGS-84 as the datum/ ellipsoid for the entire world except for the United States in which mapping and charting products will reference GRS-80 as the ellipsoid and NAD 83 as the datum. 2-15  Datum to Datum Shifts a.  All datums are defined relative to WGS-84.  For this reason, transformations between datums are performed from and to WGS-84.  In other words, when converting from Local Datum 1 to Local Datum 2 we must first transform Datum 1 to the WGS-84 and then transform the WGS-84 datum to Datum 2. b.  To develop datum shift parameters, coordinates on both datums at each of one or more physical locations must be known.  Typically, for shifts from a local datum to WGS 84, the WGS 84 coordinates were derived from Doppler satellite observations over points with already existing local datum coordinates. c.  Several methods of datum transformation are available, a few are listed below.     1.  Seven Parameter Model:  this geometric transformation model assumes that the origins of the two coordinate systems are offset from each other, that the axes are not parallel, and that there is a scale difference between the two datums.  Data from at least three well-spaced positions are needed to derive a seven parameter geometric transformation.  The seven para- meters used come from differences in the local and WGS 84 cartesian coordinates; there are three axis rotation parameters, a scale change, and three origin shift parameters ( ).  The origin shift DX , DY , DZ parameters are the coordinates of the origin of the local reference ellipsoid in the WGS 84 cartesian coordinate system.  See Figure 2-12.  Use of the Seven Parameter method is proscribed by STANAG 2211 for some applications in Europe and England.  It is considered more accurate than the five parameter model. WGS 84 Y-AXIS LOCAL ELLIPSOID X Y Z Figure 2-12  Origin Shift Parameters    2.  Five Parameter Model:  this model considers only the relative sizes of the ellipsoids and the offset differences in their origins.  The five parameters used in this model are the difference in the semi-major axes       ( ), the difference in flattening ( ), and the Da Df x 10⁴ three origin shift parameters ( ).  Origin DX , DY , DZ shift parameters are the coordinates of the origin of the local reference ellipsoid in the WGS 84 cartesian coordinate system.  See Figure 2-12.  This model is used in the computation of the Standard Molodensky DRAFT 2-6